Wide-angle polarizers with refractively reduced internal transmission angles

ABSTRACT

The usable range of incidence angles for electromagnetic wave polarizers using arrays of polarizer elements is increased by introduction of a dielectric medium having a dielectric constant large enough to reduce the angle of wave incidence upon the polarizer elements. For example, arrays of 45 degree inclined meander-line polarizer elements are encased in a dielectric medium having a dielectric constant of about 3. The polarizer includes impedance matching layers at the surfaces of the dielectric medium to reduce reflections at those surfaces. The resulting polarizer is indicated to be usable to reciprocally convert an incident polarization to a desired polarization (e.g., from linear to circular polarization) for waves with incidence angles from zero to 70 degrees in any plane.

This invention relates to polarizers usable with antennas and, moreparticularly, to polarizers capable of changing the polarization of anelectromagnetic wave from linear to circular for a wide range ofincidence angles, such as from zero to 70 degrees.

BACKGROUND OF THE INVENTION

There have previously been described polarizers for changingpolarization from linear to circular in operation with electromagneticwaves having a frequency within a frequency band and having an angle ofincidence within a range of angles. However, the usable incidence anglerange of prior polarizers has been limited. For example, alinearly-polarized phased-array antenna may be arranged toelectronically scan a radiated beam to any angle from zero to 70 degreesoff broadside in any plane. Conversion of such linear polarization tocircular polarization may be accomplished by a polarizer placed in frontof the phased-array, however the performance of prior polarizers hasdegraded substantially over such a range of incidence angles.

More specifically, prior designs of circular polarizers may incorporateseveral spaced arrays of susceptance elements which are oriented at 45degrees to an incident linear polarization for broadside incidence of anincident wave (i.e., a zero degree angle of incidence). However, atlarger angles of incidence the polarizer elements will no longer have anorientation close to 45 degrees relative to the electric field vector ofthe incident wave. As a result, the polarizer performance degrades asthe angle of incidence increases (for example, the axial ratioincreases, so that the resulting polarization is no longer circular) andthe polarizer becomes unusable beyond a limited range of incidenceangles. Thus, performance of a typical prior such polarizer may degraderapidly beyond a zero to 35 degree angle of incidence range. Also, thesusceptance of such polarizer elements changes as the incidence angle ischanged. These changes in susceptance, which are likely to be differentfor E-plane incidence and H-plane incidence, also limit the usableincidence angle range for prior polarizers.

Basic wide-band linear to circular polarizer concepts were described byD. S. Lerner in "A Wave Polarization Converter for CircularPolarization", IEEE Trans. Antennas and Propagation, Vol. AP-13, pp.3-7, Jan. 1965. Further developments of meander-line elements for use insuch polarizers were described by Young, Robinson and Hacking in"Meander-Line Polarizer", IEEE Trans. Antennas and Propagation, Vol.AP-21, pp. 376-378, May 1973 and by Chu and Lee in "Analytical Model ofa Multilayered Meander-Line Polarizer Plate with Normal and ObliquePlane-Wave Incidence", IEEE Trans Antennas and Propagation, Vol AP-35,pp. 652-661, June 1987. The latter two articles discuss the theory anddesign of meander-lines, which are polarization changing elements in theform of continuous zig-zag conductive patterns supported on thindielectric sheets. As is well known, such polarizer elements appearessentially capacitive for an incident electric field perpendicular tolength of such meander-lines and appear essentially inductive for anincident electric field parallel to the length of the meander-lines. Themeander-line approach can provide improved axial ratio and improvedfrequency band performance. However, as described and shown by Chu andLee, for a polarizer using known design techniques both the transmissioncoefficient and the input VSWR began to degrade rapidly for scan anglesgreater than about 30 degrees (see page 658 and FIGS. 6(a) and 6(b) ofthe referenced Chu and Lee article). In their Conclusion, at page 659Chu and Lee particularly point out that: "It is shown that because thepowers contained in the E-type and H-type modes of the incident wave arenot equal for oblique incidence, there will be degradation in axialratio when the meander-line polarizer is used in the oblique incidencecase."

FIG. 1 shows an array of polarizer elements in the form of a parallelarray 10 of meander-line elements 14 oriented at 45 degrees from thehorizontal and vertical. Polarizer element arrays of this type, formedas a thin metallic pattern, are used in prior polarizers. As describedin the references cited above, a basic metallic pattern, such as array10, mounted on one surface of a thin dielectric: support sheet hastypically been used in polarizers incorporating three or more of sucharray sheets maintained in spaced parallel relation by relatively thickfoam intermediate layers positioned between the array sheets. In suchconfigurations, the thin support sheets are specified to providerequired structural support of FIG. 1 type arrays, while minimizing theoperative effect of the inclusion of the dielectric materialnecessitated for such support purposes. Similarly, in such priorconfigurations, the thicker foam intermediate layers are of very lowdielectric: constant material and are also designed to minimize theoperative effect of the presence of these intermediate foam spacinglayers. Thus, in the types of prior polarizers, as described, the arraysof polarizer elements (e.g., the meander-lines 14) are intended toproduce the desired polarization change, and the support sheets and foamspacers are intended to have only minimal effects in the operation ofthe polarizer. As noted above, as the angle of incidence of an incidentwave increases beyond a limited angular range, the performance of suchprior polarizers rapidly degrades.

It is therefore an object of this invention to provide improvedpolarizers and, particularly, such polarizers usable with phased-arrayantennas to provide polarization conversion (e.g., linear to circular,vertical to horizontal, etc.) over a wide range of incidence angles.

Additional objects are to provide polarizers capable of performance overa wider range of incidence angles than prior devices, or capable ofimproved performance over a range of incidence angles within which priordevices are operable, or both.

Further objects are to provide antenna systems incorporating wide-anglepolarizers, and new and improved polarizers which avoid disadvantages orlimitations of prior devices.

SUMMARY OF THE INVENTION

In accordance with the invention, in an antenna for radiating a scannedbeam with a predetermined polarization and including an array ofradiating elements arranged for providing a linearly polarized radiatedbeam at a scan angle from broadside, a polarizer includes a dielectricmedium at least one-quarter wavelength thick at a frequency in anoperating frequency band and having a dielectric constant of at leasttwo. The polarizer is positioned in front of the array of radiatingelements for transmitting such radiated beam with an angle oftransmission within the dielectric medium which is smaller than the scanangle of the radiated beam. The polarizer also includes polarizerelement means, positioned within the dielectric medium at an orientationangle relative to the electric field vector of the radiated beam in thedielectric medium, for changing the polarization of the radiated beamfrom the linear polarization to the predetermined polarization. Alsoincluded are a first impedance-matching layer contiguous to a first sideof the dielectric medium facing toward the array of radiating elementsand a second impedance-matching layer contiguous to a second side of thedielectric medium facing away from the array of radiating elements, forreducing reflections of the radiated beam at such first and second sidesof the dielectric medium. The polarizer is arranged to cause a wavetransmitted within the dielectric medium to be incident upon thepolarizer element means at an angle smaller than such scan angle forreciprocally changing polarization of signals radiated from and receivedby the array of radiating elements.

Also in accordance with the invention, a method for changing thepolarization of an electromagnetic wave incident at an incidence angle,comprises the steps of:

(a) passing the electromagnetic wave through a first layer of materialhaving a first dielectric constant to a contiguous surface of adielectric medium having a second dielectric constant higher than suchfirst dielectric constant, the first layer being arranged to reducereflections of such wave at the contiguous surface over a range ofincidence angles;

(b) passing such electromagnetic wave from the first layer of materialinto the dielectric medium to transmit such wave within the dielectricmedium with a reduced angle of transmission, relative to the incidenceangle of such wave;

(c) changing the polarization of such electromagnetic wave byinteraction of the reduced angle wave with polarization elementspositioned within the dielectric medium; and

(d) passing such electromagnetic wave from a second surface of thedielectric medium, after such interaction with the polarizationelements, to a contiguous second layer of material havingcharacteristics similar to the first layer of material so as to reducereflections at the second surface of the dielectric medium.

Polarizers and methods in accordance with the invention are thusreciprocally operable to change the polarization (e.g., linear tocircular and vice versa) of electromagnetic waves incident over anincidence angle range, which is enhanced by said reduced angle oftransmission within said dielectric medium.

For a better understanding of the invention, together with other andfurther objects, reference is made to the accompanying drawings and thescope of the invention will be pointed out in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an array of meander-line polarizer elements.

FIG. 2 is a sectional side-view of polarizer in accordance with theinvention, which utilizes polarizer element arrays of the type shown inFIG. 1.

FIG. 3 is a simplified side-view of an antenna in accordance with theinvention, including a phased array of dipole elements and a polarizer.

FIGS. 4A and 4B are equivalent circuits useful in describing a FIG. 2type polarizer.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 2, there is shown a view of a portion of apolarizer 16 constructed in accordance with the invention. FIG. 2equally represents both a side, cross-sectional view of the polarizerportion and a top, cross-sectional view of the portion of polarizer 16.As will be described, the polarizer 16 comprises a plurality ofpolarizer element arrays, such as array 10 of FIG. 1, enclosed withindielectric material, so that FIG. 1 can be considered to represent botha front view and a mirror-reversed back view of polarizer 16 (assumingthat an enclosed element array could be viewed through the intermediateportions of dielectric material, which will be described). As shown inFIG. 2, polarizer 16 includes a dielectric medium 18 having a thickness20, which may typically exceed one-half wavelength at a frequency in anoperating frequency band. References to wavelength will normally referto free-space wavelength at a design frequency in an intended operatingfrequency band, unless otherwise noted. An important characteristic ofdielectric medium 18 is that it has a dielectric constant "K" which issignificantly higher than the dielectric constant K=1 for free space. Adielectric constant K=3 is a typical value for dielectric medium 18 inthe illustrated embodiment of the invention. In other arrangements thedielectric constant of dielectric medium 18 may typically have a valueof K=2 or greater.

The FIG. 2 polarizer also includes polarizer element means 10, 11 and 12positioned within the dielectric medium 18, for changing thepolarization of an incident wave from linear to circular polarization,for example. Polarizer element means 10 in FIG. 2 may comprise an arrayof meander-line elements 14 (such as shown in FIG. 1) positioned at anorientation angle of 45 degrees relative to the nominal direction of theelectric field vector of an incident wave as transmitted within thedielectric medium 18 (e.g., a vertically polarized wave). The "nominal"direction of the electric field vector is defined for this purpose asthe direction of such vector when the electromagnetic wave is incidentat a zero degree angle of incidence, recognizing that the actualdirection of the electric field vector of a scanned beam, relative to ameander-line element, will depend upon the specific scan angle andresulting angle of transmission of the wave in the dielectric medium.This is a cause of the "oblique incidence" degradation experienced inthe above-cited article. In the FIG. 2 embodiment, element means 12 is ameander-line element array identical to element array 10 and elementmeans 11 is a meander-line element array which is similar to elementarrays 10 and 12, but whose dimensions are chosen for polarizationchanging effectiveness when used in combination with arrays 10 and 12.The actual configurations and dimensions for meander-line element arraysfor particular embodiments can be determined by individuals skilled inthis field using known design techniques, once they have a understandingof the invention. In the FIG. 2 embodiment, the element arrays 10, 11and 12 are supported within dielectric medium 18 in a parallelconfiguration equally spaced by dimension 22, which may desirably beapproximately equal to one-quarter wavelength divided by the square rootof K at a frequency in an operating frequency band. With anunderstanding of the invention, it will be apparent to workers skilledin this field that the combination of element arrays 10, 11 and 12 anddielectric medium 18 can be implemented in a variety of ways, includingplacement of conductive patterns on layers of dielectric material whichare then combined or adhered together to effectively provide asubstantially homogeneous and continuous medium 18 with the arrays 10,11 and 12 supported within. In particular embodiments, the elementarrays may be formed on thin sheets of dielectric material of dielectricconstant higher or lower than the dielectric constant of medium 18, withthe dielectric constant of medium 18 chosen to provide the describedoperative result.

The polarizer, as shown in FIG. 2, further includes a firstimpedance-matching layer 24 contiguous to a first side of the dielectricmedium 18 and a second impedance matching layer 26 contiguous to asecond side of the dielectric medium 18 facing away from layer 24. For awave incident at an incidence angle off broadside (i.e., notperpendicular to the left or right side of polarizer 16 in FIG. 2)reflections will tend to occur at the surface of a dielectric mediumwhich represents the interface between air (having a dielectric constantK=1) and a dielectric medium having a significantly higher dielectricconstant, such as K=3 for example. Such reflections are significantlyreduced over an operating frequency band by provision ofimpedance-matching layers 24 and 26 having appropriately selectedthicknesses and dielectric constants, which in many cases will beidentical for the two layers 24 and 26. In the FIG. 2 embodiment, ifdielectric medium 18 has a dielectric constant K=3, impedance matchinglayers 24 and 26 may comprise sections of dielectric material having adielectric constant of about K=1.5 and thicknesses 28 and 30 typicallyon the order of 0.3 wavelength at a frequency in an operating frequencyband. More particularly, for use with a dielectric medium 18 having adielectric constant K=3, the thickness 28 of matching layer 24 may bedetermined as follows relative to a wavelength in an operating frequencyband: ##EQU1## Where K_(m) is the dielectric constant of the matchinglayer 24 (e.g., 1.5) and Θ_(m) is the transmission angle within layer 24for a selected angle of incidence (e.g., 45 degrees for a 60 degreeincidence angle and a 1.5 dielectric constant). This results in adimension 28 thickness of 0.29 wavelength for a non-reflective match atthe 60 degree incidence angle, which provided excellent results over thedesired zero to 70 degree incidence angle range. In other embodiments,layers 24 and 26 may each be a composite of multiple layers of materialof different thickness or dielectric constant, or both, or other knowntechniques may be employed to provide the desired impedance matchingeffect at the surfaces of dielectric medium 18.

A particular design of a FIG. 2 type polarizer includes threemeander-line element arrays 10, 11 and 12, with spacings 22 of 0.16wavelength, positioned within a medium 18 having a dielectric constantK=2.94. A bonding film having a dielectric constant of about 2.9 is usedto bond array-bearing sections of dielectric material to form adielectric medium 18 as shown in FIG. 2, which is substantiallyhomogeneous in this example. Matching layers 24 and 26, formed of singlesections of material having a dielectric: constant K=1.5, approximately,and thickness of 0.29 wavelengths, are bonded to the opposite faces ofmedium 18 by use of the same bonding film. The thickness 20 of thedielectric: medium 18, which is 0.667 wavelength in this example, isgenerally not a critical dimension, but may typically be thick enough toextend the surfaces of medium 18 outward beyond the arrays 10 and 12sufficiently to avoid effects of near-field interactions involving thedielectric interface (e.g., 18/24 interface) and the element arrays 10and 12. Analysis shows this polarizer to provide very good performancein a predetermined operating frequency band within a range of 20 to 45GHz for angles of wave incidence from zero to 70 degrees in any plane(i.e., incidence angles to 70 degrees in any lateral direction frombroadside).

In other arrangements, polarizer elements such as linear conductors,unconnected rectangular elements such as described in the Lernerarticle, or having other forms may be substituted for meander-lineelements as described and polarizers may include more or less than thethree arrays of elements as used in the described example. In polarizersincorporating only one or two polarizer element arrays, the requiredthickness of dielectric medium 18 may be significantly less than the0.667 wavelength thickness described (e.g., thickness 20 may be of theorder of one-quarter wavelength).

With respect to the operation of polarizers in accordance with theinvention, one key aspect is the inclusion of a dielectric medium 18having a dielectric constant high enough to significantly change theperformance of arrays of polarizer elements in the context of largeangles of incidence of an incident wave. FIG. 3 shows a side view of anarray of linearly-polarized dipoles 34 and associated circular polarizer36. Dipole array 34 represents a side view of rows and columns ofdipoles fed as a phased array. In use, the surface of polarizer 36closest to array 34 acts as a wave-entry surface during transmission ofan electromagnetic wave which exits from the other surface of polarizer36. During reception, the wave-entry and wave-exit surfaces arereversed, with the polarizer operating reciprocally. Known operation ofsuch a phased array antenna would permit radiation into the polarizer 36of a linearly-polarized beam scanable in any lateral direction over arange of scan angles from zero to 70 degrees. However, if circularpolarizer 36 were a typical polarizer as previously available, both theaxial ratio and insertion loss would begin to increase rapidly beyond ascan angle in excess of a value such as 35 degrees off broadside. Withinclusion of a dielectric medium 18 of higher dielectric constant inaccordance with the invention, Snell's law relating to refractiveeffects on a wave transitioning at an angle from a first medium, to asecond medium having a relatively higher dielectric constant, indicatesthat the angle of wave transmission in the second medium will bedecreased. More particularly, by application of the relationship##EQU2## it will be seen that introduction of a dielectric medium havinga dielectric constant as low as K=2 will be effective to reduce a firstangle of incidence in free space of 50 degrees, for example, to an angleof transmission within the medium of approximately 33 degrees. Thus, ona simplified analysis, an array of polarizer elements which provideefficient polarization conversion only up to an angle of 33 degrees,could operate efficiently for incidence angles to 50 degrees if thepolarizer elements are encased in a dielectric medium having adielectric constant K=2, in accordance with the invention. Of course,larger dielectric constant mediums can further extend the operableangular range so that a free space incidence angle of 70 degrees becomesa transmission angle of only 33 degrees in a dielectric medium having adielectric constant of K=3. In the design of a polarizer, the dimensionsof an array of meander-line elements may require some adjustment to takeinto account operation of the array within the dielectric medium.

On a further analytical level, the circular polarization performance foran incident wave that is linearly polarized is dependent upon therelative effects produced upon the E.sub.⊥ electric field vectorcomponent which is perpendicular to the element axis as compared withthe E.sub.∥ electric field vector component which is orthogonal to theE.sub.⊥ component and is nominally parallel to the element axis.Ideally, such parallel and perpendicular electric field components haveand maintain a ratio of unity (i.e., 1), as occurs at broadsideincidence when there is a 45 degree angle between the incident electricfield vector and the axis of the meander-line elements. In this case, ifthe polarizer elements shift the phase of one electric field componentrelative to the other by 90 degrees, the linearly polarized incidentwave will have its polarization changed to perfect circularpolarization. Actually, the two electric field components do notmaintain a unity ratio in practice as the incidence angle departs frombroadside incidence.. When the 45 degree orientation exists forbroadside incidence, the following relationships indicate the change inthe E.sub.∥ to E.sub.⊥ magnitude ratio that occurs as the incidenceangle increases: ##EQU3##

Where Θ_(OH) and Θ_(OE) are the angles of incidence in free spacemeasured off broadside in the H and E planes, respectively, and K is thedielectric constant of the dielectric medium 18 in which the polarizerelements are embedded.

It will be seen that in the absence of a dielectric medium (i.e., K=1) alarge angle of incidence (70 degrees, for example) will cause theparallel and perpendicular electric vector components to have a ratiosubstantially different from unity. This will cause a poor axial ratioand large insertion loss. However, with inclusion of a dielectric mediumhaving a substantial dielectric: constant (K=3, for example) the ratioof the parallel and perpendicular components remains close to unity,even for an angle of incidence of 70 degrees. This enables the axialratio to remain close to unity and insertion loss of the polarizer toremain small.

FIGS. 4A and 4B show simplified equivalent circuits for the FIG. 2 typepolarizer for which exemplary dimensions and dielectric constants weregiven above. FIG. 4A indicates, for the E.sub.∥ component, the designvalues of susceptance B of the embedded elements relative to the freespace admittance Y_(o) for each of the polarizer arrays 10, 11 and 12 ofFIG. 2. Similarly, FIG. 4B indicates such design values for the E.sub.⊥component. As noted, analysis of this polarizer design showed very goodaxial ratio and insertion loss performance for angles of wave incidencefrom broadside to 70 degrees off broadside. It will be appreciated that,while the invention has been described particularly in the context ofreciprocally changing between linear and circular polarizations, theinvention is also applicable to polarizers providing other changes inpolarization.

While there have been described the currently preferred embodiments ofthe invention, those skilled in the art will recognize that other andfurther modifications and variations may be made without departing fromthe invention and it is intended to claim all such modifications as fallwithin the scope of the invention.

What is claimed is:
 1. In an antenna for radiating a scanned beam with apredetermined polarization and including an array of radiating elementsarranged for providing a linearly polarized radiated beam at a scanangle from broadside, a polarizer comprising:a dielectric medium, atleast one-quarter wavelength thick at a frequency in an operatingfrequency band and having a dielectric constant of at least two,positioned in front of said array of radiating elements for transmittingsaid radiated beam with an angle of transmission within said dielectricmedium which is smaller than said scan angle of said radiated beam;polarizer element means, positioned within said dielectric medium at anorientation angle relative to the electric field vector of said radiatedbeam in said dielectric medium, for changing the polarization of saidradiated beam from said linear polarization to said predeterminedpolarization; and a first impedance-matching layer contiguous to a firstside of said dielectric medium facing toward said array of radiatingelements and a second impedance-matching layer contiguous to a secondside of said dielectric medium facing away from said array of radiatingelements, for reducing reflections of said radiated beam at said firstand second sides of said dielectric medium; said polarizer beingarranged to cause a wave transmitted within said dielectric medium to beincident upon said polarizer element means at an angle smaller than saidscan angle for reciprocally changing polarization of signals radiatedfrom and received by said array of radiating elements.
 2. An antennahaving a polarizer as in claim 1, wherein said polarizer element meanscomprise planar arrays of meander-line elements supported within saiddielectric medium.
 3. An antenna having a polarizer as in claim 1,wherein said orientation angle is nominally 45 degrees and saidpredetermined polarization is circular polarization.
 4. An antennahaving a polarizer as in claim 1, wherein said first side of saiddielectric medium is planar and is positioned normal to the broadsidebeam centerline of said radiated beam.
 5. An antenna having a polarizeras in claim 1, wherein said dielectric medium comprises substantiallyhomogenous dielectric material having a dielectric constant of at least2.5 enclosing a plurality of spaced arrays of conductive polarizerelements.
 6. An antenna having a polarizer as in claim 1, wherein saidradiating elements are arranged for providing a radiated beam scannableover a range of scan angles from broadside to 70 degrees off broadsidein all planes.
 7. An electromagnetic wave polarizer, operable with anelectromagnetic wave incident upon a wave-entry surface of saidpolarizer at an entry angle within a range of incidence angles,comprising:a first impedance-matching layer, having a wave-entry surfaceand a first dielectric constant, for reducing reflections of saidelectromagnetic wave; a dielectric medium, contiguous to a secondsurface of said first impedance-matching layer and having a thickness ofat least one-quarter wavelength at a frequency in an operating frequencyband and a second dielectric constant which is greater than said firstdielectric constant and is at least 2, for transmitting saidelectromagnetic wave with an angle of transmission within saiddielectric medium which is smaller than said entry angle as a result ofrefractive effects; polarizer element means, positioned within saiddielectric medium at an orientation angle relative to the nominaldirection of the electric field vector of said electromagnetic wave insaid dielectric medium, for changing the polarization of saidelectromagnetic wave; and a second impedance-matching layer, contiguousto a side of said dielectric medium facing away from said firstimpedance-matching layer and having a third dielectric constant which islower than said second dielectric constant and a wave-exit surface, forreducing reflections of said electromagnetic wave; said polarizer beingarranged to cause said electromagnetic wave to be incident upon saidpolarizer element means at an angle smaller than said entry angle and tooperate reciprocally so that said wave-exit and wave-entry surfaces arealso respectively usable as wave-entry and wave-exit surfaces.
 8. Apolarizer as in claim 7, wherein said first and secondimpedance-matching layers are similar sheets of a dielectric: materialhaving a dielectric constant between one and said second dielectricconstant of said dielectric medium.
 9. A polarizer as in claim 7,wherein said dielectric medium comprises substantially homogeneousdielectric material having a dielectric constant of at least 2.5enclosing and supporting a plurality of spaced arrays of polarizerelements.
 10. A polarizer as in claim 9, wherein said polarizer elementsare meander-line elements oriented at 45 degrees, relative to saidnominal direction of said electric field vector of said electromagneticwave in said dielectric medium, for changing the polarization of anincident wave from linear to circular.
 11. A polarizer, usable withincident electromagnetic waves having angles of incidence which mayexceed a limited angular range, comprising:polarizer element means,including a plurality of polarizer elements, for providing a desiredpolarization change for incident waves having angles of incidence withinsaid limited angular range; dielectric means, enclosing and supportingsaid polarizer elements and having a dielectric constant of at leasttwo, for providing a medium having a dielectric constant effective tocause refractive effects reducing the transmission angle of an incidentwave from an angle of incidence exceeding said limited angular range toan angle of transmission in said dielectric means which is within saidlimited angular range; and impedance matching means, coupled to incidentwave entry and exit surfaces of said dielectric means, for reducingreflections of said incident wave at said entry and exit surfaces ofsaid dielectric means; said polarizer being arranged so that a wavetransmitted within said dielectric means is incident upon said polarizerelements at an angle smaller than the angle of incidence of said waveupon said polarizer.
 12. A polarizer as in claim 11, wherein saiddielectric means has a thickness of at least one-quarter wavelength at afrequency in an operating frequency band.
 13. A polarizer as in claim11, wherein said polarizer elements comprise meander-line conductivepatterns positioned within said dielectric means, which comprises asubstantially homogeneous dielectric medium at least one-quarterwavelength thick at a frequency in an operating frequency band.
 14. Apolarizer as in claim 13, wherein said meander-line conductive patternshave a 45 degree orientation relative to the nominal direction of theelectric field vector of said incident wave in said dielectric means.15. A method for changing the polarization of an electromagnetic waveincident at an incidence angle, comprising the steps of:(a) passing anelectromagnetic wave through a first layer of material having a firstdielectric constant to a contiguous surface of a dielectric mediumhaving a second dielectric constant higher than said first dielectricconstant, said first layer being arranged to reduce reflections of saidwave at said contiguous surface over a range of incidence angles; (b)passing said electromagnetic wave from said first layer of material intosaid dielectric medium to transmit said wave within said dielectricmedium with a reduced angle of transmission, relative to said incidenceangle of said wave; (c) changing the polarization of saidelectromagnetic wave by interaction of said wave with polarizationelements positioned within said dielectric medium; and (d) passing saidelectromagnetic wave from a second surface of said dielectric medium,after said interaction with said polarization elements, to a contiguoussecond layer of material having characteristics similar to said firstlayer of material so as to reduce reflections at said second surface ofsaid dielectric medium; said method being reciprocally operable tochange the polarization of electromagnetic waves incident over anincidence angle range which is enhanced by effects of said reduced angleof transmission within said dielectric medium.
 16. A method as in claim15, wherein step (b) comprises passing said electromagnetic wave into adielectric: medium having a dielectric constant of at least
 2. 17. Amethod as in claim 15, wherein step (b) comprises passing saidelectromagnetic wave into a dielectric medium having a thickness of atleast one-quarter wavelength at a frequency in an operating frequencyband.
 18. A method as in claim 15, wherein step (a) comprises passingsaid electromagnetic wave into said first layer with the electric fieldvector of said wave aligned at a nominally 45 degree angle relative tosaid polarization elements positioned within said dielectric medium, forchanging linear polarization to circular polarization.
 19. A method asin claim 15, wherein step (b) comprises passing said electromagneticwave into a substantially homogeneous dielectric medium, at leastthree-eighths wavelength thick at a frequency in an operating frequencyband, which encloses and supports a plurality of spaced arrays of saidpolarization elements.